For low power memory, charge pumps are required to generate voltages over the power supply voltage or below the ground bias voltage to reduce leakage current and improve access speed. Therefore, the current efficiency of charge pumps is important when the power or current consumption is considered. Existing multi-phase charge pumps with single-rail bias control, however, may not have desirable charge transferring ability. If higher voltages are required, the existing design may become more complicated and a larger size will be consumed at each stage.